The present invention relates to a superheterodyne receiver with an information-transmission circuit, a local oscillator tunable with a dc voltage, and a digitally adjustable tuning arrangement which is connected to the output of the local oscillator and contains a frequency divider with numerically adjustable division ratio, a device for digitally adjusting the frequency divider, and a frequency comparator developing at its output, in the vicinity of a fixed reference frequency, a dc voltage dependent on the input frequency.
The ever increasing number of transmitters and the more and more closely spaced transmitter frequencies necessitate more accurate tuning in the receivers. Analogously operating, continuously adjustable tuning arrangements, particularly of the kind used in radio and television receivers so far, increasingly fail to meet these requirements and this so much the more if these tuning arrangements are operated by unskilled persons, which is generally the case with radio and television receivers. With digitally adjustable tuning devices, however, if the tuning data, e.g. the transmitter frequency, is known, the tuning operations can be performed by any unskilled person with ease and the necessary accuracy.
Superheterodyne receivers of the kind referred to above are known in the art. The output of the local oscillator for the mixer in the information transmission circuit of the known receiver is followed by a preselection counter organized in decades and operating as a digitally adjustable frequency divider whose division ratio is adjusted with a stepping switch. The output frequency of the frequency divider is fed to a phase-comparison discriminator in which it is compared with the frequency of a reference oscillator.
The local oscillator is controlled with the tuning voltage generated by the frequency deviation. A considerable disadvantage of this known tuning arrangement is that the control voltage of the discriminator is simultaneously the tuning voltage for the local oscillator. Thus, accurate frequency setting is achieved only in the center of the adjustable frequency range, while at the ends, particularly at the higher-frequency end of the frequency range, major deviations from the nominal frequency occur because of the control-voltage response.